Zoom lens system

ABSTRACT

A zoom lens system permitting photographing at definite magnification levels comprising a first lens unit and a second lens unit, and equipped with a first mechanism for varying f and β so as to keep f/β nearly constant and a second mechanism for varying, independently of the first mechanism, the airspace between the first lens unit and the second lens unit when the reference symbol f represents total focal length of the first lens unit and the reference symbol β designates total lateral magnification of the second lens unit. Said zoom lens system is equipped with simple mechanisms and permits photographing objects located at distances within a broad range at optional magnification levels within a certain range.

This is a continuation of application Ser. No. 07/469,795 filed on Jan.22, 1990, which was abandoned upon the filing hereof, and which was acontinuation of Ser. No. 07/251,396 filed Sept. 30, 1988, now abandoned.

BACKGROUND OF THE INVENTION

a) Field of the Invention

The present invention relates to a zoom lens system, and morespecifically a zoom lens system permitting photographing at definitemagnification levels to provide images at definite magnification levelsdespite variation of distances to photographed objects.

b) Description of the Prior Art

For photographing objects which are coming nearer or going farther withcinematography cameras, video cameras and so on, it has conventionallybeen necessary to perform zooming and focusing simultaneously andcontinuously for photographing images of the objects while keepingconstant image sizes on the imaging planes. It is difficult, even withan automatic focusing camera, to perform the operations smoothly forsuch photographing at a constant magnification level. Especially, highlyskilled techniques are required for such photographing with the TVcameras for professional and semi-professional photographers or lowlypriced video cameras equipped with no automatic focusing mechanisms.

Further, photographing must be performed at a definite magnificationlevel when academic documents are to be prepared by macro photographingwith a still camera. When a macro lens system having a single focallength is to be used for photographing for this purpose, outdoorphotographing is difficult and indoor photographing is inconvenientsince it is necessary to keep a constant distance from an object to animage thereof. If a zoom lens system is used in this case, it ispossible to photograph an object at a constant size despite variation ofdistance from the object to an image thereof, however inconvenience iscaused since focusing and zooming must be performed simultaneously.Furthermore, focal length of the ordinary zoom lens system may be variedwhen it is focused on an object located at a different distance andcomplicated operations are required for actual photographing at adefinite magnification level.

Moreover, when lens systems are used as eyes of a robot, it is necessaryto control the robot on the basis of accurate comprehension ofphotographing magnification. In case of the ordinary zoom lens systemsused as the eyes of a robot, it is necessary to read focused and zoomedconditions on an encoder and determine magnification levels throughcomputations for controlling the robot. That is to say, the robot mustbe controlled while carrying out computations on the basis of twoconditions of focusing and zooming.

A method to solve the above-described problem is disclosed by JapaneseExamined Published Patent Application No. 10050/61. This methodcomprises determination of compensator movement relative to variator asa predetermined movement different from the movement for zooming andphotographing at a definite magnification level while performingfocusing on an object to be photographed regardless of distance to theobject to be photographed.

However, this method requires a cam (hereinafter referred to as "cam forphotographing at a definite magnification level") which performscompensator movement different from the ordinary zooming movement, andshape of the cam is determined for a specific magnification level. Theshape of the cam must be changed for a different magnification level. Inorder to make it possible to vary magnification level continuously forphotographing at definite magnification levels, it is thereforenecessary to change shape of the cam continuously. For making itpossible to vary photographing magnification level continuously andmechanically by the above-described method, it is therefore necessary toadopt a complicated mechanism which will be highly priced even with thecurrent electronic control technology.

In addition, it is possible to perform photographing at a nearlydefinite magnification levels with a relatively lowly priced lens systemby using a single cam for photographing at a definite magnificationlevel and performing photographing utilizing depth of field. However,photographing at definite magnification levels will be limited within anarrow range and magnification range will be very narrow especially formacro photographing.

SUMMARY OF THE INVENTION

A primary object of the present invention is to provide a zoom lenssystem having a simple mechanisms, and permitting photographing objectslocated at photographing distance within a broad range at optionalmagnification levels within a certain range.

The zoom lens system according to the present invention is characterizedin that said zoom lens system comprises at least two lens units disposedon a common optical axis, that the lens unit disposed or arranged on theobject side out of these lens units is movable along the optical axisand is capable of varying focal length (this lens unit will hereinafterbe referred to as the first lens unit), that the lens unit disposed orarranged on the image side of said first lens unit is freely movablealong the optical axis (this lens unit will hereinafter be referred toas the second lens unit), and that said zoom lens system is so designedas to keep a substantially constant value of f/β regardless of variationof focal length f and variation of β caused by the movement of thesecond lens unit when focal length of the first lens unit is representedby f and imaging magnification level of the second lens unit isdesignated by β.

In a preferred formation of the present invention, the zoom lens systemaccording to the present invention is equipped with a first mechanismfor varying f and β, and a second mechanism for varying the airspacereserved between the first lens unit and the second lens unitindependently of the operation of the first mechanism.

Principle of the zoom lens system according to the present inventionwill be outlined with reference to FIG. 1 through FIG. 4.

In these drawings illustrating a lens system, G corresponds to the firstlens unit and preferably consists of a plural number of lens components.Further, G₂ corresponds to the second lens unit an preferably consistsalso of a plural number of lens components.

In these drawings, the reference symbol I represents an imaging plane,the reference symbols P_(W) and P_(T) designate the positions conjugatewith I with regard to the second lens unit G₂ at the wide position andthe tele position respectively, the reference symbols a_(W), A_(T),a_(W) ' and a_(T) ' denote distances between the first lens unit G₁ andP_(W) or P_(T), the reference symbols b_(W) and b_(T) representdistances between P_(W) or P_(T) and the second lens unit G₂, thereference symbols c_(W) and c_(T) designate distances between the secondlens unit G₂ and the photographing plane I, and the reference symbolsO_(W) and O_(T) denote the positions conjugate with I with regard to thezoom lens system as a whole.

FIG. 1 shows a condition where the zoom lens system is focused on anobject located at infinite distance at the wide position set by theordinary zooming operation, FIG. 2 shows a condition where the zoom lenssystem is focused on an object located at infinite distance at the teleposition set by the ordinary zooming operation, FIG. 3 illustrates acondition where the first lens unit G₁ is moved as a whole for adistance x=a'_(W) -a_(W) (>0) toward the object side from the conditionshown in FIG. 1 and FIG. 4 illustrates a condition where the first lensunit G₁ is moved as a whole for a distance x=a'_(T) -a_(T) (>0) towardthe object side from the condition shown in FIG. 2.

Now, let us calculate photographing magnification levels M₃ and M₄ ofthe zoom lens system as a whole in the conditions shown in FIG. 3 andFIG. 4 respectively.

Let us represent imaging magnification for the object point O_(W) of thefirst lens unit G₁ by β_(1W) in FIG. 3. Since the rear focal point ofthe first lens unit G₁ is coincident with the point p_(W) conjugate withI with regard to the second lens unit G₂ in FIG. 1, the imagingmagnification β_(1W) is calculated as follows:

    β.sub.1W =-x/f.sub.W

wherein the reference symbol f_(w) represents the focal length of thefirst lens unit G, at the wide position.

Further, since the photographing magnification M₃ of the zoom lenssystem is equal to β_(1W) ×β_(W), it is given by the following equation:

    M.sub.3 =-xβ.sub.W /f.sub.W

wherein the reference symbol β_(w) represents the magnification level ofthe second lens unit G₂ at the wide position.

Similarly, let us designate imaging magnification for the object pointO_(T) of the first lens unit G₁ by β_(1T) in FIG. 4. Since the rearfocal point of the first lens unit G₁ is coincident with P_(T) in FIG.2, the imaging magnification β_(1T) is expressed as follows:

    β.sub.1T =-x/f.sub.T

wherein the reference symbol f_(t) represents the focal length of thefirst lens unit G₁ at the tele-position.

Since the imaging magnification level M₄ of the zoom lens system isequal to β_(1T) ×β_(T), it is given by the following equation:

    M.sub.4 =-xβ.sub.T /f.sub.T

Of course, as f/β is substantially a constant, the following equationsare also true:

    M.sub.4 =-xβ/f and x=-M.sub.f /β.

wherein the reference symbol β_(t) represents the magnification level ofthe second lens unit G₂ at the tele-position.

Let us now determine the condition for obtaining M₃ =M₄. From therelationship of -xβ_(W) /f_(W) =-xβ_(T) /F_(T), we obtain:

    f.sub.T /f.sub.W =β.sub.T /β.sub.W

Accordingly, imaging magnification for a focused object is set at aconstant level when the focal length f_(A) of the first lens unit G₁ andthe lateral magnification β_(A) of the second lens unit for the pointP_(A) conjugate with I with regard to the second lens unit G₂ satisfyf_(A) /f_(W) =β_(A) /β_(W) in an optional condition between the wideposition and the tele position, and the distance x between the rearfocal point of the first lens unit G₁ and P_(A) is kept at a constantvalue.

Further, photographing at a definite magnification level is madepossible by equipping the zoom lens system with a first mechanism whichis similar to the ordinary cam so composed as to move the lens units insuch a manner that a constant ratio of f/β is obtained between the focallength f of the first lens unit G₁ and the lateral magnification β ofthe second lens unit G₂, and a second mechanism for varying the airspacebetween the first lens unit G₁ and in addition to the ordinary zoomingcam (the first mechanism), with the second lens unit G₂ independently ofthe ordinary zooming and the ordinary focusing. Moreover, thesemechanisms may be simple mechanisms each having a single cam orhelicoid, and are manufacturable at low cost.

In practice, it is conceivable to adopt a method to shift the first lensunit G₁ and the second lens unit G₂ so as to obtain a targetmagnification level, i.e., to set a magnification level with the secondmechanism and adjust distance from an object to an image with the firstmechanism, or another method to set a photographing magnification levelby varying the distance between the frist lens unit G₁ and the secondlens unit G₂ with the second mechanism while focusing the zoom lenssystem on an object to be used as a standard of image size with thefirst mechanism, and then focus the zoom lens system on another objectlocated at a different distance with the first mechanism.

Out of the methods described above, the latter requires rathercomplicated operations since the first and second mechanisms must beoperated in a certain correlation. However, the operations can besimplified by using an automatic focusing mechanism in combination withthe second mechanism at the stage to set a photographing magnificationlevel since such a means make it sufficient to perform the variation ofthe distance between the first lens unit G₁ and the second lens unit G₂.

In the foregoing description, switching from x≠0 to x=0 (x: variation ofthe distance between the first lens unit G₁ and the second lens unit G₂)i.e., switching from the ordinary photographing condition to thephotographing condition at a definite magnification level is performedwith the zoom lens system focused on an object located at infinitedistance. It is not necessary to set x=0 at the switching point.

However, the switching may be performed in a condition where the lenssystem is focused on an object located at any distance so long as f/β iskept constant after the switching. Further, when the first lens unit G₁is designed as a variable refractive index element which is made of anelastic material and has a shape variable to change the refractive indexthereof, it is possible to vary the focal length f even with a singleelement. In an actual lens system, image size may be varied due todistortion even at the same paraxial magnification level. In such acase, it is possible to cancel the variation due to distortion at therequired portion of the image by adequately varying value of f/β.Assuming that D_(hW) represents distortion at an image height of h whenthe focal length f has a value of f_(W) and that D_(hA) designatesdistortion at the image height of h when the focal length f has a valueof f_(A), it is sufficient to adjust the value f_(A) /β_(A) of f/β atthe focal length of f_(A) so as to be (f_(W) /β_(W))×{(1-D_(hA) /100)}for cancelling the distortion at the image height of h when the focallength is f_(A).

When f/β is adjusted to f_(A) /β_(A) after a target value f_(A) /β_(A)of f/β is determined taking distortion in to consideration, distortionis varied. This variation of distortion results in a slight variation oftarget value f_(A) /β_(A). However, f_(A) /β_(A) is converged to acertain value by repeating the adjustment and a required precision canbe obtained by repeating the adjustment. Further, even when the value off/β is varied slightly, it is possible to perform photographing nearlyat a definite magnification level if required precision is not so high.

Since ±10% is considered as limits of magnification variation allowablefor the ordinary photographing, the following values may be consideredas limits for photographing at a definite magnification level:

    1.22≧|f/β|.sub.max / |f/β|.sub.min ≧1

wherein the reference symbols |f/β|_(max) and |f/β|_(min) representmaximum and minimum values of f/β respectively.

Furthermore, a definite magnification level is kept constant in thefocused condition so long as the distance FP between therear focal pointF of the first lens unit G₁ and the point P conjugate with the imagingplane with regard to the second lens unit G₂ is kept constant duringphotographing at the definite magnification level despite the movementof the first mechanism. In an actual lens system, however, magnificationlevel may be varied due to spherical aberration and chromaticaberration. Accordingly, in order to obtain an accurate imagingmagnification level, it is necessary to set FP and distance SK from thesecond lens unit G₂ to the imaging plane taking into consideration thedeviations of the first lens unit G₁ and the second lens unit G₂ due tothe aberrations. Assuming that adjustment degrees of these distances arerepresented by ΔFP_(a) and ΔSK_(a) respectively, these degrees cannot bedetermined uniquely since they are related in a complicated manner toaberration conditions, pupil position, magnification levels of the firstlens unit G₁ and the second lens unit G₂ , etc. However, it isconsidered that these degrees are nearly proportional to thelongitudinal aberration δ_(a) on the imaging plane and within a rangefrom some fractions to several times of the longitudinal aberrationδ_(a).

The zoom lens system according to the present invention is so designedas to be always set at a definite magnification level in a focusedcondition thereof and focusing precision must be enhanced to stabilizethe magnification level.

When focusing precision is low, magnification level is unstable forphotographing at a definite magnification level. Assuming that ΔFPrepresents precision of value of FP determined depending on focusingprecision in such a case, it is insignificant to set variation of FPsmaller than ΔFP and it is sufficient to keep variation of FP on theorder of ΔFP. When focusing precision in the direction along the opticalaxis on the imaging plane is designated by δ_(F), value of ΔFP is on theorder of δF/β2.

In addition to the mode of operation described above, it is possible toadopt the operation mode disclosed by Japanese Examined Published PatentApplication No. 10050/61. That is to say, a focused condition can beobtained by adjusting in such a manner that an image of an object iskept at a constant size when the distance from the object to an imagethereof is varied. In this case, it is sufficient to select precision ofthe distance FP satisfying depth of field. Error ΔFP_(l) in this case ison the order of δL/β2 when depth of field is represented by δ_(L).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 through FIG. 4 show illustrations descriptive of the principle ofthe zoom lens system according to the present invention; and FIG. 5shows a scheme illustrating composition of an embodiment of the zoomlens system according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Now an embodiment of the zoom lens system according to the presentinvention will be described. FIG. 5 shows composition of an embodimentof the present invention. This embodiment comprises three lenscomponents: a first lens component L₁ and a second lens component L₂composing a first lens unit G₁ whereas a third lens component L₃composing a second lens unit G₂. The embodiment has the numerical datashown in the following Tables 1 through 3:

                  TABLE 1                                                         ______________________________________                                        f.sub.1 = 50, f.sub.2 = -20, f.sub.3 = 33                                     Focused on infinite distance                                                  f.sub.t                                                                            S     e.sub.1 e.sub.2                                                                             S'    f      β.sub.1                                                                      .sub.β                                                                         IO                            ______________________________________                                        35   ∞                                                                              1.429  32    66    -35    0   -1    ∞                       49   ∞                                                                              5.853  24.325                                                                              72.046                                                                              -41.413                                                                              0   -1.183                                                                              ∞                       63   ∞                                                                              8.704  18.814                                                                              77.274                                                                              -46.957                                                                              0   -1.342                                                                              ∞                       77   ∞                                                                             10.737  14.483                                                                              81.947                                                                              -51.913                                                                              0   -1.483                                                                              ∞                       91   ∞                                                                             12.281  10.891                                                                              86.211                                                                              -56.436                                                                              0   -1.612                                                                              ∞                       105  ∞                                                                             13.504   7.804                                                                              90.158                                                                              -60.622                                                                              0   -1.732                                                                              ∞                       ______________________________________                                    

                                      TABLE 2                                     __________________________________________________________________________    Photographing at a definite magnification of - 0.02×                    β.sub.t                                                                      S     e.sub.1                                                                           e.sub.2                                                                           S'  f    β.sub.1                                                                      β                                                                            IO                                         __________________________________________________________________________    -0.02                                                                             -1712.5                                                                              1.429                                                                            32.7                                                                              66  -35  0.02                                                                              -1  1812.629                                   -0.02                                                                             -2396.469                                                                            5.853                                                                            25.025                                                                            72.046                                                                            -41.413                                                                            0.0169                                                                            -1.183                                                                            2499.393                                   -0.02                                                                             -3082.606                                                                            8.704                                                                            19.514                                                                            77.274                                                                            -46.957                                                                            0.0149                                                                            -1.342                                                                            3188.098                                   -0.02                                                                             -3770.217                                                                           10.737                                                                            15.183                                                                            81.947                                                                            -51.913                                                                            0.0135                                                                            -1.483                                                                            3878.084                                   -0.02                                                                             -4458.910                                                                           12.281                                                                            11.591                                                                            86.211                                                                            -56.436                                                                            0.0124                                                                            -1.612                                                                            4568.994                                   -0.02                                                                             -5148.446                                                                           13.504                                                                             8.504                                                                            90.158                                                                            -60.622                                                                            0.0115                                                                            -1.732                                                                            5260.611                                   __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________    Photographing at a definite magnification of - 0.3×                     β.sub.t                                                                     S    e.sub.1                                                                           e.sub.2                                                                           S'  f    β.sub.1                                                                       β                                                                             IO                                         __________________________________________________________________________    -0.3                                                                              -79.167                                                                            1.429                                                                            42.5                                                                              66  -35  +0.3 -1   189.095                                    -0.3                                                                             -109.802                                                                            5.853                                                                            34.825                                                                            72.046                                                                            -41.413                                                                            0.2535                                                                              -1.1832                                                                           222.526                                    -0.3                                                                             -142.606                                                                            8.704                                                                            29.314                                                                            77.274                                                                            -46.957                                                                            0.2236                                                                             -1.342                                                                             257.898                                    -0.3                                                                             -176.883                                                                           10.737                                                                            24.983                                                                            81.947                                                                            -51.913                                                                            0.2023                                                                             -1.483                                                                             294.550                                    -0.3                                                                             -212.244                                                                           12.281                                                                            21.391                                                                            86.211                                                                            -56.436                                                                            0.1861                                                                             -1.612                                                                             332.127                                    -0.3                                                                             -248.446                                                                           13.504                                                                            18.304                                                                            90.158                                                                            -60.622                                                                            0.1732                                                                             -1.732                                                                             370.411                                    __________________________________________________________________________

In these tables, the reference symbols f₁, f₂ and f₃ represent focallengths of the fist lens component L₁, the second lens component L₂ andthe third lens component L₃ respectively, the reference symbols e₁ ande₂ designate distance between the principal points of the first lenscomponent L₁ and the second lens component L₂, and distance between thesecond lens component L₂ and the third lens component L₃ respectively,the reference symbols S and S' denote distance from the first lenscomponent L₁ to the object point and distance from the third lenscomponent L₃ to the image point respectively, the reference symbol frepresents total focal length of the first lens component L₂ and thesecond lens component L₂ (the first lens unit G₁), the reference symbolβ₁ designates total lateral magnification of the first lens component L₁and the second lens component L₂ (the first lens unit G₁), the referencesymbol β represents lateral magnification of the third lens component L₃(the second lens unit G₂), the reference symbol f_(t) represents focallength of the zoom lens system as a whole, the reference symbol β_(t)represents total lateral magnification of the zoom lens system as awhole, and the reference symbol IO designates distance from the objectto the image.

Out of the tables shown above, Table 1 shows movements of the individuallens components for the ordinary zooming with the zoom lens systemfocused on an object located at infinite distance.

As is understood from this table, value of f/β is kept constant sincethe ordinary zooming cam has the function of the first mechanism whichkeeps value of f/β constant.

Further, Tables 2 and 3 list conditions where magnification levels forphotographing at definite magnification levels are set at -0.02×and-0.3×respectively.

As is understood from these tables, the distance e₂ is variedindependently of the ordinary zooming. Variations of the distance are0.7 at the magnification level of -0.02×and 10.5 at the magnificationlevel of -0.3×.

The second mechanism is used for varying the distance e₂ independentlyof the ordinary zooming as described above.

Under the present circumstance where advanced electronic controltechnique is available, it is possible to use a focusing mechanismindependent of the above-described first mechanism and the secondmechanism. Any one of the lens components can be moved by the focusingmechanism but it will be the simplest to move the first lens component.When any type of focusing mechanism is adopted as the third mechanism,the requirement of constant f/β is made unsatisfied by focusing with thethird mechanism. For photographing at a definite magnification levelwith the zoom lens system equipped with the third mechanism, it istherefore necessary to keep the zoom lens system focused on an objectlocated at infinite distance with the third mechanism.

Owing to the first and second mechanisms described above, the zoom lenssystem according to the present invention permits photographingaccurately at definite magnification levels, adjustment within a broadrange, has simple composition and is manufacturable at a low cost. Inother words, the second mechanism permits setting magnification levelfor photographing at definite magnification levels and, in addition,continuously varying magnification level. On the other hand, the firstmechanism permits focusing for photographing at definite magnificationlevels with a photographing magnification kept at a constant level.Furthermore, the zoom lens system according to the present inventionallows to determine a photographing magnification level uniquely withthe second mechanism, and accordingly reduces amount of computations andfacilitates control when the zoom lens system is used as an eye of anindustrial robot since the zoom lens system can ordinarily be focusedwith the first mechanism while kept at a definite magnification leveland the second mechanism is operated only when magnification level is tobe varied.

I claim:
 1. A zoom lens system permitting photogrpahing at a constantimaging magnification level regardless of variation of an object point,said lens system comprising:a first lens unit and a second lens unitbeing arranged on a common optical axis in the order recited from theobject side; said first lens unit having a variable focal length andbeing movable as a whole along the optical axis; said second lens unitbeing movable along the optical axis; a moving distance x of said firstlens unit relative to said second lens unit being given as x=-M₄ ·f/βregardless of variation of said focal lens f and variation of theimaging magification level β of said second lens unit being caused bythe movement thereof, the magnification in level of said zoom lenssystem being kept substantially constant regardless of the variation ofobject point and being represented by M₄ ; and f/β having asubstantially constant value regardless of the variation of said focallength f and the variation of said imaging magnification level β of saidsecond lens unit caused by the movement thereof.
 2. A zoom lensaccording to claim 1 wherein said first lens unit is movable along theoptical axis so as to keep, regardless of the movement of said secondlens unit, a substantially constant distance between the point conjugatewith a predetermined image plane of said zoom lens system as a wholewith regard to the lens units taken as a whole arranged on the imageside of said first lens unit and the rear focal point of said first lensunit.
 3. A zoom lens system permitting photographing at definitemagnification levels according to claim 2 comprising a third focusingmechanism operating independently of the second mechanism.